The aim of this book is to provide an introduction to the science of cancer for those coming to the topic for the first time – be they students or graduates or post-doctoral scientists moving into the field of oncology. That is, to paint a picture of what we think happens to cells and molecules in the making of cancers, how it bears on diagnosis and prognosis, and where the science is taking us in terms of treatment. To scientists and non-scientists alike, cancer can seem almost the ultimately daunting subject. It’s true that cancers are the most complex diseases that afflict us and it is arguable that no two cases are identical, if all the biochemical changes involved are identified. However, while at some point we will face that problem – indeed we shall see that its very complexity may offer some advantage in the therapeutic battle against the disease – it’s now clear that the underlying principles by which cancers arise are remarkably consistent and conceptually straightforward.